Jag längtar efter dig när du reser.

Because längta efter is the normal Swedish expression for to long for / to miss someone or something.

• längta by itself means to long / to yearn
• efter is the preposition that usually goes with it when you say what or who you long for

So:

• Jag längtar. = I’m longing / yearning.
• Jag längtar efter dig. = I long for you / I miss you.

This is something you mostly just learn as a fixed combination: längta efter någon/något.

Because dig is the object form of du.

Compare:

• du = you, as the subject
• dig = you, as the object

In this sentence:

• Jag is the subject
• längtar efter is the verb expression
• dig is the object of the preposition efter

So Swedish works like English here:

• you travel → du reser
• I miss you → jag längtar efter dig

Here när means when.

It introduces the time clause när du reser, which tells us when the main action happens.

So the structure is:

• Jag längtar efter dig = main clause
• när du reser = time clause

Swedish när is very commonly used for when in this kind of sentence.

Swedish often uses the present tense where English might use:

• the simple present
• the present progressive
• even a future meaning, depending on context

So du reser can mean things like:

• you travel
• you are travelling
• you’re away travelling
• sometimes you will travel, if the context makes that clear

In this sentence, the exact English wording depends on context, but Swedish does not need a special continuous form like are travelling.

Not exactly, though it is often translated that way.

längta efter has a sense of:

• longing for
• yearning for
• strongly wanting someone’s presence

So it can sound a bit more emotional or wistful than plain I miss you, depending on context.

In everyday translation, though, I miss you when you travel is a very natural meaning.

Yes, absolutely. Jag saknar dig när du reser is very natural Swedish.

There is a small nuance:

• sakna = to miss
• längta efter = to long for / miss with a stronger feeling of yearning

So:

• Jag saknar dig is often the most direct equivalent of I miss you
• Jag längtar efter dig can sound a bit more emotional or poetic

Both are correct, but they are not always identical in tone.

In när du reser, the word order is:

• när = conjunction
• du = subject
• reser = verb

So it is simply:

när + subject + verb

That is normal for a subordinate clause in Swedish.

The full sentence is:

• Jag = subject
• längtar = finite verb
• efter dig = prepositional phrase
• när du reser = subordinate time clause

One useful thing to notice: Swedish main clauses usually follow V2 word order, but subordinate clauses like när du reser do not.

It can mean either one, depending on context.

It may describe:

• a repeated situation: I miss you when you travel
• a current situation: I miss you while you’re away traveling
• a planned/future situation: I’ll miss you when you travel

Swedish present tense is flexible, so context tells you which meaning is intended.

Because Swedish does not need one here.

The clause när du reser already expresses the time relationship by itself. English might sometimes say:

• when you travel
• when you’re travelling
• while you’re away

But Swedish can often express all of that with just när + present tense, especially if the meaning is already clear from context.

The most straightforward negation is:

Jag längtar inte efter dig när du reser.

That means I don’t miss/long for you when you travel.

Notice where inte goes in the main clause:

• Jag längtar inte efter dig

If you want to negate the subordinate clause instead, you would say:

Jag längtar efter dig när du inte reser.

That means I miss you when you’re not travelling, which is a different meaning.

So placement of inte matters a lot.

It is grammatically neutral, but the choice of längtar efter makes it sound a bit emotional.

• Jag längtar efter dig sounds warm, personal, and heartfelt.
• It is not overly formal.
• It can be romantic, but it does not have to be.

You could say it to:

• a partner
• a family member
• someone you feel close to

So the grammar is neutral, but the wording carries feeling.